glial is correct, the technologies behind AJAX (JavaScript, CSS, XML, JSON, etc.) have been around for a long time. It seems that these techniques just needed a name (coined, as it was, by Jesse James Garrett) and someone brave enough and high profile enough (Google Maps) to blaze the path for the rest of us mortals. That was the first app I remember seeing that really took advantage of AJAX in a smart way. The first time I used it, I knew the game had changed.
Used smartly, AJAX can make a web application much more responsive by making web pages interactive while sending much smaller payloads back and forth. Used dumbly, it can break a lot of aspects of the user experience.